bpo-40280: Enable assertions in Emscripten debug builds

[emmatyping]

Setting is documented here: https://emscripten.org/docs/porting/Debugging.html#compiler-settings

ASSERTIONS=1 is used to enable runtime checks for common memory allocation errors (e.g. writing more memory than was allocated). It also defines how Emscripten should handle errors in program flow. The value can be set to ASSERTIONS=2 in order to run additional tests.
ASSERTIONS=1 is enabled by default. Assertions are turned off for optimized code (-O1 and above).

These are useful for catching things like OOMs and such.

https://bugs.python.org/issue40280

[github-actions]

This PR is stale because it has been open for 30 days with no activity.

[emmatyping]

This is now included in the Makefile so I think this can just be closed!